This invention relates to molten aluminum containing molten salts and more particularly it relates to a method of removing finely divided particles of molten salts from molten aluminum.
In melting aluminum and transferring it, a considerable amount of impurities is often introduced into the melt. These impurities include gas (typically hydrogen from moisture in the atmosphere) non-metallic impurities (mostly being derived from the aluminum oxide film on the melt charge or that which forms on the surface of molten aluminum as it is melted and transported) together with sodium or other metallic impurities which can be introduced in the smelting process. It is important that these impurities be reduced to the minimum levels possible. Gases in the solidified metal produce a number of problems in fabricating and using aluminum products as does the presence of oxides. The gas content and oxide content seem to be related in that oxide particles tend to nucleate the formation of hydrogen filled discontinuities.
The presence of sodium interferes with certain fabrication procedures, especially hot rolling where any significant amount causes severe edge cracking during hot rolling reductions. This is especially significant in alloys containing magnesium, for instance 2 to 10% Mg, where edge cracking becomes very serious.
One example of difficulty in reducing the sodium content by chlorination is that the magnesium present in most aluminum alloy melts is ordinarily reacted simultaneously. This occurs even though chlorine, or the reaction product of chlorine with aluminum, aluminum chloride, react with sodium preferentially over magnesium at equilibrium conditions. From considerations of chemical reaction equilibria and the law of mass section, chlorine released in the melt would first be expected to largely form aluminum chloride because aluminum is by far the major component of the melt. Next in sequence, some of the aluminum chloride may encounter and react with magnesium in the melt to form magnesium chloride because magnesium is usually more concentrated than the other melt components capable of reacting with aluminum chloride. Finally, if contact with the metal is maintained long enough, the magnesium or aluminum chlorides encounter the trace amounts of sodium and react to form the final equilibrium product, sodium chloride. Rate of chlorination and magnesium concentration are factors determining how far and how rapidly reaction proceeds through this sequence to the final equilibrium product, sodium chloride. At commonly used chlorination rates, final equilibrium is difficult to achieve without using contact times which are unacceptable in a continuous commercial process. Accordingly, it has been difficult to achieve extremely low sodium levels under commercial production plant conditions which require comparatively large amounts of molten metal to be treated rather rapidly.
One of the difficulties in achieving extremely low levels of salts is that even after fluxing and filtering some of the molten salt formed can remain in very fine particle form or droplets suspended in the melt and as such becomes extremely difficult to separate by flotation or sedimentation in flowing streams of molten aluminum. Such suspended salt dispersions are of such a nature as to pass through molten metal filters and end up in the aluminum ingot with their attendant problems.
Thus, there is a great need for a process suitable for removing finely divided salt dispersion in molten aluminum. The present invention provides such a process wherein particles or droplets of salt, e.g., smaller than 225 microns, for example, can be effectively coalesced or amalgamated into droplets and which can then be brought to the surface by floatation and removed from the melt.